This invention relates to an ultrasonic testing method and apparatus for detecting flaws of balls for structural members, and more particularly to an ultrasonic testing method for accurate detection of minute flaws in the surface and the inside of balls for structural members, wherein at least two probes are used.
As a testing method for detecting flaws of steel materials, steel plates, forgings, etc., there has been known an immersion-type ultrasonic testing method for detecting flaws in which water is used as a medium for transmitting ultrasonic waves.
In application of the conventional ultrasonic testing method for detecting flaws in products to be or being used under severe conditions, a focus-type probe obtained by attaching a concave resin lens to an oscillator or by forming the oscillator itself in a concave shape has been used to detect flaws several hundreds of micrometers in size, thereby achieving a higher reliability.
On the other hand, research have been conducted on the use of ceramics for bearing members and the like, which are required to have high reliability. Ceramics are brittle materials, and, especially in the case of balls for structural members such as ceramic balls used for bearings which have been being developed in recent years, the presence of flaws such as pores, foreign matter and cracks in a portion of the ball might cause a concentration of stress on that portion, resulting in breakage. The larger the flaws become and the nearer the flaws to the surface of the ball are, the greater the effect of the flaws becomes. Therefore, a testing method with high resolution has been desired for detection of such flaws of the ceramic products.
The present inventors previously proposed, in Japanese Patent Application Laid-Open (KOKAI) No. 63-243751 (1988) (which corresponds to U.S. patent application Ser. No. 172,244) and Japanese Patent Application Laid-Open (KOKAI) No. 1-219554 (1989) (which corresponds to U.S. patent application Ser. No. 07/311,041), an ultrasonic testing method and apparatus for detecting flaws of balls in which a focus-type ultrasonic probe is used with the focus located on the surface of the ball and with the angle of incidence of the ultrasonic waves set equal to the critical angle of the transverse wave, thereby detecting accurately the flaws present in the surface of the ball and the flaws present at a depth of 2 mm from the surface;
Though a higher detection accuracy than that in the prior art is achieved by the ultrasonic testing method for detecting flaws previously proposed by the present inventors, most of the incident ultrasonic waves are reflected by the surface of the ball or are converted into a surface wave and, therefore, only a part of the ultrasonic waves propagate into the inside of the ball. Accordingly, in the previously proposed method, the detection sensitivity for the internal flaws has been lower than that for surface flaws. Because detection of flaws with higher accuracy requires a finer detection pitch, the method has required a long time for performing flaw detection for the entire surface of the ball with higher accuracy.